The present invention relates to a process for producing a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst. In more particular, it relates to a process for producing a particle diameter-controlled molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst having a desired particle diameter distribution.
Further, the present invention relates to a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst obtained by the above-mentioned process, the use of the catalyst, and a process for producing acrylonitrile or methacrylonitrile which uses the catalyst.
The use of a molybdenum-bismuth-containing metal oxide fluidized bed catalyst in ammoxidation of propylene, isobutene and/or tertiary butanol is disclosed, for example, in JP-B-36-3563, JP-B-36-5870, JP-B-38-17967, JP-B-39-3670, JP-B-39-10111, JP-B-42-7774, JP-B-50-64191, JP-B-47-27490, JP-B-54-22795 and JP-B-60-36812.
With regard to a process for producing a fluidized bed catalyst having a controlled particle diameter distribution, proposals have been made in JP-A-52-140490 as to an iron-antimony type oxide fluidized bed catalyst and in JP-A-5-261301 as to a vanadium-phosphorus type oxide fluidized bed catalyst. However, no effective method has been proposed as to a process for producing a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst.
It is well known that, in a fluidized bed reaction, in order to keep catalyst particles in a good fluidizing condition thereby to make the reaction proceed efficiently, it is very important that the physical property, particularly the particle diameter distribution, of the catalyst is appropriate.
In producing a fluidized bed catalyst, it is usually conducted for obtaining a catalyst having a desired particle diameter distribution to control the conditions of spray drying. By such a means alone, however, it is quite difficult to produce the intended catalyst, and catalyst particles having unnecessarily large or small diameters tend to be inevitably formed. When the amount of catalyst particles having small diameters is too large, in a fluidized bed reaction, such catalyst particles tend to fly away during the reaction to cause an increase in the amount of the catalyst to be replenished. In particular, catalyst particles with a diameter of 20 xcexcm or less are apt to fly out of the system. When the amount of catalyst particles having a large diameter is too large, the fluidizing property of the catalyst tends to deteriorate to worsen the result of the reaction.
Furthermore, even when a catalyst having an appropriate particle diameter distribution is used in a fluidized bed reaction, the catalyst particles with small diameters gradually fly away during the reaction to shift the particle diameters toward larger ones. In such a case, measure is commonly taken in which a catalyst containing much of fine particles is replenished so that the catalyst in the reactor may keep an appropriate particle diameter distribution. In preparing the catalyst used for replenishing mentioned above, it is difficult to obtain a catalyst of the desired particle diameter distribution by mere control of the spray drying conditions. Therefore, it is economically advantageous to use a supplementary catalyst having controlled particle diameters produced by the combination of the control of spray drying conditions with the removal by classification of particles with a diameter of 20 xcexcm or less, which are apt to fly away. Further, it is favorable if the fine particles removed by classification can be reused as the catalyst starting material.
However, no such methods have been disclosed with regard to a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst. Moreover, even when such a known method is applied as such to the present catalyst system, satisfactory results cannot be obtained because the activity and physical property of the catalyst are adversely affected. It is estimated that, in a catalyst of the present system, when the catalyst particles removed by classification and having particle diameters outside the desired range are added to a slurry before spray drying, the solution composition-precipitation composition ratio becomes different from that in the initial catalyst slurry, which may exert a great influence on the performance characteristics of the catalyst. Under such situations, development of a process has been awaited which can produce a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst having controlled particle diameters economically efficiently while keeping good catalytic characteristics.
After extensive study, the present inventors have found that, in a process for producing a metal oxide fluidized bed catalyst containing molybdenum, bismuth, iron and silica as the essential constituents of the catalyst component, by separating dried particulate products with particle diameters outside the desired particle diameter range from spherical particles obtained by spray drying operation, pulverizing the dried products to particle diameters of 10 xcexcm or less, then mixing the pulverized products into the slurry at a stage prior to spray drying within the range of 50% by weight or less (based on the oxides of the completed catalyst), spray-drying the resulting mixture, and subjecting the spray-dried product to classificatio, catalyst particles having diameters outside the desired particle diameter range can be effectively utilized and, as a whole, a practically useful molybdenum-bismuth-containing metal oxide fluidized bed catalyst which has a high strength, particularly excellent abrasion resistance, and moreover sufficient activity can be produced in a reasonable way.
Thus, according to the present invention, there is provided a process for producing a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst containing molybdenum, bismuth, iron and silica as essential components and having a controlled particle diameter, said process comprising the step of spray-drying a slurry containing catalyst components to effect granulation, which comprises the steps of
[I] spray-drying a slurry containing catalyst components,
[II] subjecting the dry particles obtained by the spray drying to classification to separate dry particles having a particle diameter outside a desired range, and feeding dry particles having a particle diameter within the desired range to the subsequent calcination step,
[III] pulverizing the dry particles having a particle diameter outside the desired range so as to have a particle diameter of 10 xcexcm or less to obtain a pulverized product, and
[IV] mixing the pulverized product into the slurry containing catalyst components at any desired stage prior to the spray drying so as to be in the range of not more than 50% by weight in terms of oxides based on oxides of a completed catalyst obtained after the spray drying and the calcination.
The metal oxide fluidized bed catalyst produced by the process for producing a metal oxide fluidized bed catalyst of the present invention described above is preferably a catalyst having a composition represented by the formula
MoaBibFecQdReXfYgOh(SiO2)i
wherein Mo, Bi, Fe and O respectively represent molybdenum, bismuth, iron and oxygen, Q represents at least one element selected from the group consisting of nickel, cobalt, magnesium, chromium, manganese and zinc, R represents at least one element selected from the group consisting of beryllium, phosphorus, boron, arsenic, selenium, lithium, sodium, potassium, rubidium, cesium, thallium and tellurium, X represents at least one element selected from the group consisting of vanadium, tungsten, yttrium, lanthanum, zirconium, hafnium, niobium, tantalum, aluminum, calcium, strontium, barium, lead, copper, cadmium, gallium, indium, germanium, antimony, tin and cerium, Y represents at least one element selected from the group consisting of praseodymium, neodymium, samarium, europium, gadolinium, thorium, uranium, rhenium, ruthenium, rhodium, palladium, osmium, iridium, platinum, silver and gold, and SiO2 represents silica; suffixes a, b, c, d, e, f, g, h and i represent atomic ratios of the respective elements, provided that when a=10, then 0.1xe2x89xa6bxe2x89xa65, 0,1xe2x89xa6cxe2x89xa610, 0xe2x89xa6dxe2x89xa68, 0xe2x89xa6exe2x89xa63, 0xe2x89xa6fxe2x89xa68, 0xe2x89xa6gxe2x89xa62 and 10xe2x89xa6ixe2x89xa6200; and h is the number of oxygen atoms necessary for satisfying valencies of the above respective components.
Further, according to the present invention, there are provided a molybdenum-bismuth-iron-containing metal oxide fluidized bed catalyst obtained by the above-mentioned process, a process for using the catalyst, and a process for producing acrylonitrile or methacrylonitrile which uses the catalyst.
The catalyst itself having the composition represented by the formula mentioned above may be produced by any desired method, but it is particularly preferable that the respective components be closely mixed together to form one body.
The starting materials of the respective component elements for producing the catalyst may be selected from oxides or from chlorides, sulfates, nitrates, ammonium salts, carbonates, hydroxides, organic acid salts, oxyacids, oxyacid salts, heteropolyacids, heteropolyacid salts, and the mixtures thereof which can be converted into oxides by ignition. The ratios of the amounts of these materials to be used may be appropriately varied according to the composition ratios of respective elements in the final catalyst obtained.
The materials for the molybdenum component which may be used are, for example, oxides such as molybdenum trioxide; molybdic acid or its salts, such as molybdic acid, ammonium paramolybdate and ammonium metamolybdate; and heteropolyacids containing molybdenum, such as phosphomolybdic acid and silicomolybdic acid, or their salts. Preferably used is ammonium paramolybdate or ammonium metamolybdate.
The materials for the bismuth component which may be used are, for example, bismuth salts, such as bismuth nitrate, bismuth carbonate, bismuth sulfate and bismuth acetate; bismuth trioxide and metallic bismuth. These material may be used as a solid as it is, or as aqueous solution or aqueous nitric acid solution, or as a slurry of bismuth compound formed from these aqueous solutions, but it is preferable to use a nitrate, or its solution, or a slurry formed from the solution.
The materials for the iron component which may be used are, for example, ferrous oxide, ferric oxide, ferrous nitrate, ferric nitrate, iron sulfate, iron chloride, organic acid iron salts and iron hydroxide and, further, a solution obtained by dissolving metallic iron in heated nitric acid. Solutions containing an iron component may be used after pH-controlled with aqueous ammonia or the like. Preferably, ferrous nitrate or ferric nitrate is used.
As for the material for the silica component, it is convenient to use a suitable silica sol selected from those available on the market.
As for the other materials, preferably used are oxides, or nitrates, carbonates, organic acid salts, hydroxides, etc., or the mixtures thereof which can be converted into oxides by ignition; more preferably used are nitrates.
A slurry containing the catalyst components can be prepared by closely mixing the above-mentioned catalyst raw materials so as to give a desired composition. The preparation of the slurry may be done by any known methods, for example, the methods described in JP-B-37-8568, JP-B-57-49253, JP-B-54-12913, JP-B-51-1674, JP-A-2-59046, and JP-A-2-214543. The means for mixing the raw materials in the slurry preparation and the conditions of slurry preparation, such as temperature, pressure and atmosphere, may be set as desired.
The respective catalyst components may be mixed in successive order, in the form of solid or solution, into silica sol or water. It is also possible to conduct pH control and/or heat treatment in the course of the slurry preparation step. The solutions of the respective catalyst components used for the slurry preparation may be one obtained by dissolving partial, plural components beforehand or one which has been further subjected to pH control and heat treatment. These operations exert no particular effect on the effect of the present invention.
In controlling the pH, the iron component can be prevented from precipitating by making a chelating agent coexist in the solution containing the iron component. The chelating agent which may be used is, for example, ethylenediaminetetraacetic acid, lactic acid, citric acid, tartaric acid and gluconic acid. In making an aqueous solution containing an iron ion and a chelating agent, it is preferable to dissolve these raw materials in acid or water.
The slurry thus prepared is then subjected to spray drying, whereby substantially spherical particles are formed. The spray drying conditions are not particularly limited. Spray driers of pressure nozzle type, two fluid nozzle type and rotating disk type, etc. may be used for the spray drying. The concentration of the slurry subjected to spray drying is preferably about 10-50% by weight in terms of the oxides of elements constituting the catalyst.
The spray drying temperature also is not particularly limited but, when the temperature is extremely high, care must be taken because in addition to the general tendency of the shape of the catalyst becoming worse, in some cases in the application of the present example, the spray-dried product tends to be difficulty pulverized. For example, the spray drying can be conducted at a temperatures in the range of 100-500xc2x0 C., or in the range of 150-350xc2x0 C. The pressure and the atmosphere in the spray drying may be set as desired.
From the spherical particles formed by the spray drying are separated extra fine particles and/or coarse particles which are not suited to practical use. Cakes which form at the time of spray drying (owing to the deposit or the like on the inner wall of the spray drying apparatus) may also be regarded as coarse particles. The particle diameters of the extra fine particles and/or the coarse particles to be separated vary depending on the intended reactors and reaction conditions and also on the particle density of the catalyst. Therefore, it is preferable to determine the particle diameters to be separated by taking the properties and the using conditions of the catalyst into consideration. The conditions of the separation, for example, means for separation, temperature, pressure and atmosphere in the separation may be set as desired.
The range of diameters of the particles to be separated is preferably not more than 10-20 xcexcm for extra fine particles and not less than 100-300 xcexcm for coarse particles. In particular, for extra fine particles, it is preferable to separate particles of 20 xcexcm or less and, for coarse particles, it is preferable to separate those of 200 xcexcm or more, more preferably those of 150 xcexcm or more. The term xe2x80x9cparticle diameterxe2x80x9d herein refers not to the average particle diameter of the whole particles but to the particle diameter of individual particles.
When the separation is necessary, separately a classifier may be used. Known classifiers, e.g., sieves, cyclones and pneumatic classifiers, may be used. However, particularly when the production of the above-mentioned replenishing catalyst is intended, a catalyst is desired which has a narrow particle diameter distribution on the relatively small particle diameter side and contains neither extra fine particles nor coarse particles, so that the efficiency of particle diameter control at this time is quite important.
Separated particles with particle diameters outside the desired range are, according to necessity, pulverized by using a known grinding machine (pulverizer), such as a colloid mill, ball mill and vibrating mill. The grinding conditions, such as grinding means, temperature, pressure and atmosphere, may be set as desired. The method of wet grinding is particularly preferable. At this time, the particles may be mixed with water, or mixed with a catalyst starting material or with a slurry before spray drying. Though the particles which have undergone the spray drying step have a strength sufficient for the above-mentioned particle diameter controlling operation conducted, for example, by classification, they can be relatively easily pulverized and, according to the wet grinding method, most of the particles can be pulverized to 10 xcexcm or less in a short time.
When large particles get mingled in the slurry, the shape of the resulting completed catalyst tends to be poor. Therefore, it is preferable to pulverize most of the particles to 10 xcexcm or less, more preferably 5 xcexcm or less. More specifically, pulverization is preferably conducted until the proportion of particles of 10 xcexcm or less, preferably 5 xcexcm or less, reaches 50% by weight or more, preferably 80% by weight or more, particularly preferably 95% by weight or more. Since particles which have undergone a calcination step have a high strength and require much energy for pulverization, particle diameter control is preferably applied to spray-dried particles.
The pulverized particles thus obtained are mixed into the above-mentioned slurry before spray drying and used. The mixing conditions, e.g. mixing means, temperature, pressure and atmosphere, may be set as desired. The mixing may be done at any stages before spray drying, for example, the stage of catalyst raw material mixing, the stage of pH controlling, and before or after the stage of heat treatment. Though the mixing can be conducted in any stage, it is preferable to mix the particles into the slurry immediately before the spray drying from the viewpoints of the rationality and the reproducibility of operation.
The amount of the spray-dried, pulverized product to be mixed is preferably not more than 50% by weight, more preferably in the range of 1-50% by weight, still more preferably in the range of 2-30% by weight, in terms of oxides based on the oxides of the completed catalyst. When the pulverized product is mixed in an amount exceeding 50% by weight, the resulting ratio of solution composition to precipitation composition in the slurry differs greatly from that in the initial catalyst slurry, so that the reaction characteristics of the catalyst tend to be poor. When the mixing proportion of the catalyst particles removed by classification is 1% or less, it tends to be difficult to obtain a fluidized bed catalyst having particle diameters controlled to the desired range.
The slurry containing pulverized particles prepared as described above is then formed into substantially spherical particles by being subjected to spray drying.
The spray-dried product containing substantially no extra fine particles and/or coarse particles obtained by separating extra fine particles and/or coarse particles is then calcined to give a catalyst. The calcination conditions, e.g., calcining means, temperature, pressure and atmosphere, may be set as desired. Such a spray-dried product is calcined to give a catalyst by heat treatment in the temperature range of preferably 200xc2x0 C.-800xc2x0 C., more preferably 400xc2x0 C.-750xc2x0 C. for, e.g., 0.5-10 hours. The gas atmosphere used in the calcination may be either an oxidizing gas atmosphere containing oxygen or an inert gas atmosphere, e.g., nitrogen, but air is conveniently used. The calcination may be conducted by using, for example, a box type furnace, tunnel furnace, rotary furnace and fluidization furnace.
Though the above-mentioned metal oxide fluidized bed catalyst in the present invention is not particularly restricted so long as it has the composition represented by the above-mentioned formula, preferably the Q element is at least one element selected from the group consisting of nickel, cobalt, magnesium, chromium and manganese, the R element is at least one element selected from the group consisting of potassium, phosphorus, sodium, rubidium, cesium and tellurium, the X element is at least one element selected from the group consisting of cerium, vanadium, tungsten, lanthanum, zirconium, niobium, tantalum, aluminium, gallium, germanium, antimony and tin, and the Y element is at least one element selected from the group consisting of praseodymium, neodymium, samarium, rhenium, ruthenium, rhodium, palladium, iridium and platinum.
In the above-mentioned formula (1), preferable atomic ratios of the respective elements are, when a=10, then 0.2xe2x89xa6bxe2x89xa64, 0.2xe2x89xa6cxe2x89xa68, 0xe2x89xa6dxe2x89xa67.5, 0xe2x89xa6exe2x89xa62, 0xe2x89xa6fxe2x89xa67.5, 0xe2x89xa6gxe2x89xa61.5 and 20xe2x89xa6ixe2x89xa6150.
The catalyst produced as described above is filled in a fluidized bed reactor, and an olefin of a starting material, oxygen, ammonia, etc. are fed into the reactor, whereby an ammoxidation reaction can be effected. The starting olefin is preferably propylene or isobutene. The oxygen source preferably used is air on account of economical advantage, but air appropriately enriched with oxygen may also be used. If necessary and desired, an inert gas, such as nitrogen or steam, may also be fed to the reactor. In this way, acrylonitrile or methacrylonitrile can be obtained.
The present invention is described in detail below with reference to Examples and Comparative Examples, but the invention is in no way limited thereto. The composition ratios of the respective elements in the catalyst obtained were calculated from the amounts of the raw materials of the respective constituent elements on the assumption that the total amounts of the respective constituent elements (Mo, Bi, Fe, Q component, R component, X component, Y component and SiO2) contained in the starting materials used for catalyst preparation are incorporated as such into the catalyst.